In vivo and in vitro inhibition of the L-type calcium current in isolated guinea-pig cardiomyocytes by the immunosuppressive agent cyclosporin A.

Cyclosporin A (CsA), an immunosuppressive agent used to reduce rejection after organ transplantation, induces secondary effects in heart tissue. We have studied the effects in vivo and in vitro of CsA on L-type Ca2+ current (ICa) and the associated gating currents of isolated guinea-pig ventricular myocytes using the whole-cell patch-clamp technique. For in vivo experiments, a group of animals (n=28) was treated for 21 days by subcutaneous injection of CsA (15 mg/kg/day). Blood level of CsA was 1191+/-221 ng/ml (n=9). In cells from these animals (n=65, 19 animals), ICa was reduced to about 75% of that recorded from control cells (n=32, six animals). CsA decreased the availability of Ca2+ channels at potentials more positive than +30 mV. Isoproterenol (100 nM) was still able to increase ICa but only by 30+/-6% (n=9), whereas in control it increased ICa by 290+/-22% (n=5). Gating currents related to L-type Ca2+ channels were not altered in cells from CsA-treated animals. In in vitro experiments, CsA reduced ICa when applied directly to cardiomyocytes. CsA affected the kinetics of ICa inactivation, slowing down the rapid phase and accelerating the slow phase (n=4). The steady-state inactivation curve of ICa was shifted to more negative voltages and the degree of availability at -80 mV decreased by in vitro application of CsA. The half inactivation potential (V1/2) changed from -23+/-0.6 mV in control to -31+/-2 mV, -48+/-0.6 mV and -49+/-0.6 mV, in 1, 50 and 80 microM CsA, respectively. In these cells, the gating currents related to L-type Ca2+ channels were also not altered by CsA. CsA does not modify the Ca2+ channel density, although it induces a decrease in the beta1-adrenergic stimulation of ICa. The results are explained by a direct effect on the calcium channel inactivation of CsA and a non specific indirect effect.